Move src/ to lisa/src/ for consistency

build.sbt

lisa/src/main/scala/lisa/proven/Main.scala

lisa/src/main/scala/lisa/proven/SetTheoryLibrary.scala

lisa/src/main/scala/lisa/proven/mathematics/Mapping.scala

lisa/src/main/scala/lisa/proven/mathematics/SetTheory.scala

lisa/src/main/scala/lisa/proven/tactics/Destructors.scala

lisa/src/main/scala/lisa/proven/tactics/ProofTactics.scala

...scala/lisa/proven/tactics/SimplePropositionalSolver.scala

lisa/src/test/scala/lisa/proven/InitialProofsTests.scala

lisa/src/test/scala/lisa/proven/SimpleProverTests.scala